MARIUPOL, Ukraine, May 10 (Reuters) - Tossing his spanner to the ground, the man in a red beret and bomber jacket lowered himself into the captured armoured personnel carrier (APC) and began to turn and tilt its 50 mm gun.

Behind him, barricades of garbage bins and wooden pallets blocked every entrance to the central square of the eastern port city of Mariupol, after a day of gunfire and death that took Ukraine one step closer to civil war.

Mariupol has the stench of a city spiralling out of control, where burning tyres belch black smoke into the sky and a few women in motorcycle helmets cruise the downtown clutching metal bars.

Many residents will join other mainly Russian-speakers in Ukraine's industrial southeast on Sunday in voting in a chaotic referendum on self-rule, on breaking away from a government in Kiev that wants to take the country westward, away from Moscow.

Given the scene on Saturday in the city of half a million, a vital industrial hub, the result of the vote looks increasingly irrelevant. The fight for Mariupol has moved well beyond the ballot-box.

"We don't know what happened. We came here to find out, but they're not telling us anything," said Tatiana, sister of a police officer called Mikhail who was shot dead on Friday at a police station in a leafy neighbourhood of Mariupol.

What happened was a massive assault by Ukrainian troops on a police station they said had been taken over by pro-Russian militiamen.

Estimates of the death toll ranged from seven to more than 20. Dozens were wounded. The troops opened fire on the building from automatic rifles, APCs and rocket-propelled grenades, according to television pictures captured by an ITN cameraman.

Civilians were among those caught in the crossfire. Interior Minister Arsen Avakov said those killed were "terrorists". The police chief was reported abducted.

APCs sped through the city, after retreating troops fired into the air to escape an advancing crowd. One APC broke down and was abandoned by the soldiers. It was set alight by pro-Russians on Saturday, a man standing on the carcass brandishing a bullet.

"The last time the tanks were here it was '43," he shouted, referring to Nazi Germany's occupation of Mariupol. "Now they're coming back, sent by that junta!"

AMBIGUITY

On Saturday, police officers in civilian clothes hastily removed their uniforms from the back of the smouldering police station, throwing them into the boot of a car and driving off.

"I don't know who did the shooting," said one, who refused to be named.

A man who identified himself as a commander at the station told a reporter: "I could talk to you, but someone will get shot."

Police in Ukraine's Donetsk steel and coal belt have largely stood to one side in the fight for control of the region, where gunmen seized public buildings in response to the February ouster of Moscow-backed president Viktor Yanukovich in an East-West struggle for the soul of Ukraine.

The new government in Kiev, which blames the uprising on Moscow, says the future of Ukraine's 45 million people is in Europe, but many in the east refuse to break historical ties with Russia, and are harking back to World War Two in branding the government 'fascist'.

The army, however, has not given up Mariupol, clashing almost daily over the past few days with pro-Russian militants who were driven out of City Hall on Thursday, the building set ablaze on Friday.

The city is the Donetsk region's second biggest. How the pro-Russians will conduct Sunday's referendum here is not clear.

The plebiscite is shaping up to be a sham. No outside observers have been invited. The ballot papers are little more than black-and-white printed pages that could be duplicated, and no minimum turnout has been set for the result to stand.

Voters will be asked to endorse an ambiguously phrased status that could be interpreted as anything from "self-rule" to "independence". Some hope they will be annexed by Russia, like their brethren in the Crimean peninsula in March.

National Guard troops backed by soldiers in four APCs checked cars entering and leaving the city on Saturday, on a route the ballot papers should take to the separatist electoral authorities in the regional capital Donetsk after voting ends at 10 p.m. (1900 GMT) on Sunday.

The result will bring no resolution.

"Kiev won't consider the referendum legitimate," said a pro-Russian activist who gave his name as Slavik, "but they also came to power illegitimately."

Asked where the vote was being organised in the city, he replied: "It's a secret, it's confidential. It's too dangerous." (Writing by Matt Robinson; editing by Ralph Boulton and Janet Lawrence)